JP2018077214A - X-ray image conversion screen, flat panel detector, and X-ray inspection apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray image conversion screen capable of taking an X-ray image superior in image quality in high sensitivity region, and a high functional flat panel detector including an X-ray image conversion screen and a photodetector, and an X-ray inspection apparatus capable of taking and analyzing X-ray image superior in image quality in a high sensitivity region.SOLUTION: Disclosed is an X-ray image conversion screen including: a support base plate; and a fluorescent body layer laminated over the support base plate. The phosphor layer contains a gadolinium oxysulfide phosphor having at least one selected from the group consisting of Tb, Pr, Ce, Yb, and Eu as an activator. When defining the sensitivity of the X-ray image conversion screen is x, and q 'DQE is y, in a range of sensitivity 1900-3600, the sensitivity is in an X-ray image conversion screen which satisfies y-(-29.222x+174950)≥5000.SELECTED DRAWING: Figure 1

Description

  The present invention resides in an X-ray image conversion screen, a flat panel detector, and an X-ray inspection apparatus.

Conventionally, an X-ray image using a film has been widely used in a medical field. However, since the X-ray image using the film is analog image information, the resolution is not sufficient and the storage stability is not good. Therefore, in recent years, digital systems such as computed radiography (CR) and flat panel type X-ray detectors (FPD) have been developed.
In an indirect flat panel detector (FPD: flat panel detector) that converts X-rays into visible light, an X-ray image conversion screen is used to convert X-rays into visible light. The X-ray image conversion screen includes X-ray phosphors such as thallium-activated cesium iodide (CsI: Tl) and terbium-activated gadolinium oxysulfide (Gd ₂ O ₂ S: Tb) (GOS). Depending on the line, the X-ray phosphor emits visible light, and the light emission is converted into an electrical signal by a photodetector equipped with a TFT, CCD, etc., thereby converting X-ray information into digital image information. To do.
FPDs with higher sensitivity to irradiated X-rays and high sharpness are desired. Increasing the amount of phosphor in the X-ray image conversion screen is effective for increasing the sensitivity. However, increasing the amount of phosphor increases the thickness of the X-ray image conversion screen and causes visible light emitted from the phosphor to be emitted. It becomes easier to spread and sharpness decreases. In order to suppress the influence of this light diffusion and improve the sensitivity while maintaining the sharpness, for example, Patent Document 1 discloses a method of adjusting the average particle diameter, filling rate, film thickness, etc. of the phosphor. Yes.

JP 2007-248283 A

Known factors that determine the image quality of an X-ray image taken with an X-ray inspection apparatus include contrast (gradient γ of the characteristic curve), resolution characteristics / sharpness (MTF), noise characteristics / granularity (WS), and the like. ing. However, the evaluation results by contrast, MTF, and WS do not always coincide with each other, and the image quality of the obtained X-ray image is not always satisfactory.
In recent years, attention has been focused on DQE (Detective Quantum Efficiency) detected by “sharpness / noise” as an index representing image quality. We have clarified that in the conventional technology, sensitivity and DQE are in a trade-off relationship in a specific sensitivity region.
The present invention has been made in view of the above, and provides an X-ray image conversion screen capable of capturing an X-ray image with excellent image quality in a high sensitivity region.
The present invention also provides a flat panel detector including the above-described X-ray image conversion screen and a photodetector, capable of capturing an X-ray image with excellent image quality in a high sensitivity region.
Furthermore, the present invention provides an X-ray inspection apparatus capable of capturing an X-ray image with excellent image quality in a high sensitivity region.

As a result of intensive studies, the present inventors have found that the above problems can be solved by using an X-ray image conversion screen containing a specific phosphor and having specific characteristics, and the present invention has been achieved. did.
That is, the gist of the present invention is an X-ray image conversion screen including a support substrate and a phosphor layer laminated on the support substrate, wherein the phosphor layer includes Tb, Pr, Ce, Yb. And a gadolinium oxysulfide phosphor having at least one selected from the group of Eu as an activator, and the sensitivity of the X-ray image conversion screen is x and q′DQE is y, the sensitivity is 1900 or more, 3600 In the following range, an X-ray image conversion screen satisfying the following formula (1), a flat panel detector including the X-ray image conversion screen and a photodetector, and an X-ray image inspection including the flat panel detector Exists in the device.
y − (− 29.222x + 174950) ≧ 5000 (1)
In addition, the phosphor amount per unit area in the phosphor layer is preferably 110 mg / cm ² or more.
Furthermore, the amount of phosphor per unit area in the phosphor layer is preferably 140 mg / cm ² or more.
Moreover, it is preferable that the reflectance of the surface on which the phosphor layer of the support substrate is laminated is 90% or more.
Furthermore, the reflectance of the surface on which the phosphor layer of the support substrate is laminated is preferably 95% or more.
The support substrate preferably contains at least one selected from the group consisting of titania, alumina, barium sulfate, calcium carbonate, zinc carbonate, and magnesium carbonate.
Moreover, it is preferable that the volume average particle diameter of the phosphor contained in the phosphor layer is 5 μm or more and 15 μm or less.

  The X-ray image conversion screen having the above-mentioned characteristics is, as described later in the embodiment of the invention, the type of phosphor, the particle size of the phosphor, the particle size distribution of the phosphor, and the fluorescence to the X-ray image conversion screen. It can be prepared by appropriately adjusting the coating amount of the body, the filling rate of the phosphor in the phosphor layer, the reflectance of the support substrate on which the phosphor is coated, and the like.

According to the present invention, it is possible to provide an X-ray image conversion screen capable of capturing an X-ray image with excellent image quality in a high sensitivity region.
In addition, the present invention can provide a flat panel detector including the X-ray image conversion screen and the photodetector, which can capture an X-ray image with excellent image quality in a high sensitivity region.
Furthermore, the present invention can provide an X-ray inspection apparatus capable of capturing an X-ray image with excellent image quality in a high sensitivity region.

It is sectional drawing which represented typically the structure of the flat panel detector containing the X-ray image conversion screen which concerns on one Embodiment of this invention. It is a figure which shows typically schematic structure as one Embodiment of the X-ray inspection apparatus to which this invention flat panel detector is applied. It is the figure which plotted the sensitivity and q'DQE of the X-ray image conversion screen in an experimental example. The vertical axis is q'DQE, and the horizontal axis is sensitivity.

Hereinafter, the present invention will be described with reference to embodiments and examples. However, the present invention is not limited to the following embodiments and examples, and may be arbitrarily modified without departing from the gist of the present invention. Can be implemented.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
Hereinafter, preferred configurations of an X-ray image conversion screen according to an embodiment of the present invention and a flat panel detector using the same (hereinafter sometimes referred to as “FPD”) will be described with reference to FIG. Is not limited to these.

The present invention relates to an X-ray image conversion screen comprising a support substrate and a phosphor layer laminated on the support substrate, wherein the phosphor layer is a group of Tb, Pr, Ce, Yb, Eu. And a gadolinium oxysulfide phosphor using Tb as an activator, and the sensitivity of the X-ray image conversion screen is x and q′DQE is y, and the sensitivity is 2400 or more, 3600 The X-ray image conversion screen satisfies the following formula (1) in the following range. By satisfying the following formula (1), it is possible to provide an X-ray image conversion screen capable of capturing an X-ray image with excellent image quality even in a high sensitivity region. The present invention is also a flat panel detector provided with the X-ray image conversion screen and a photodetector, and an X-ray image inspection apparatus provided with the flat panel detector. It is more preferable that the following expression (2) is satisfied from the viewpoint that an X-ray image with higher image quality can be taken.
y − (− 29.222x + 174950) ≧ 5000 (1)
y − (− 29.222x + 174950) ≧ 7000 (2)
Further, the lower limit of the sensitivity of the X-ray image conversion screen is 1900 or more, and may be 2400 or more, 2700 or more, or 2900 or more depending on the required characteristics, while the upper limit is 3600. It may be 3400 or less from the viewpoint of increasing DQE, and 3200 or less is more preferable.

  The flat panel detector 1 includes an X-ray image conversion screen 2, a photodetector 3, and a power supply unit (not shown). The X-ray image conversion screen 2 absorbs incident X-ray energy and has an electromagnetic wave with a wavelength in the range of 300 nm to 800 nm, that is, an electromagnetic wave (light) in the range from ultraviolet light to infrared light centering on visible light. Is emitted.

The X-ray image conversion screen 2 includes a support substrate 4 and a phosphor layer 5 laminated on the support substrate.
The flat panel detector 1 is obtained by adhering or bringing the light exit surface of the X-ray image conversion screen 2 and the photodetector 3 into contact with each other via the protective layer 6. The incident direction of the X-rays to the X-ray image conversion screen 2 is not particularly limited, and may be incident from the support substrate 4 side of the X-ray image conversion screen 2 or may be incident from the phosphor layer 5 side. The light emitted from the X-ray image conversion screen 2 reaches the photodetector 3, performs photoelectric conversion, and outputs it. Hereinafter, each component will be described.

[Support substrate]
Examples of the supporting substrate on which the phosphor layer is laminated include cellulose acetate, cellulose propionate, cellulose acetate butyrate, polyethylene terephthalate and other polyesters, polystyrene, polymethacrylate, polyamide, vinyl chloride-vinyl acetate copolymer, polycarbonate resin, and the like. to TiO ₂ (titania) or Al ₂ O ₃ (alumina), BaSO ₄ (barium sulfate), CaCO ₃ (calcium carbonate), ZnCO ₃ (zinc carbonate) and MgCO ₃ are dispersed a filler (such as magnesium carbonate) Paper, aluminum, etc. are used.
The reflectance of the surface of the support substrate on the side where the phosphor layers are laminated is usually 80% or more, preferably 90% or more. The X-ray image conversion screen according to the present invention is suitable for use with high-energy X-rays. In that case, the reflectance of the surface of the support substrate on which the phosphor layer is laminated is preferably 95% or more, More preferably, it is 98% or more.
The thickness of the support substrate is not particularly limited, and is usually 10 μm or more, preferably 50 μm or more, and usually 200 μm or less, preferably 100 μm or less.

[Phosphor layer]
The phosphor layer used in the present invention is a layer containing a phosphor.
(Phosphor particles)
In this embodiment, the phosphor particles absorb incident X-ray energy and emit electromagnetic waves having a wavelength of 300 nm to 800 nm, that is, electromagnetic waves (light) ranging from ultraviolet light to infrared light centering on visible light. Refers to phosphor particles.
The phosphor layer is made of gadolinium oxysulfide phosphor (Gd ₂ O ₂ S), terbium (Tb), praseodymium (Pr), cerium, as described in JP-A Nos. 2000-162394 and 2003-82347. (Ce), phosphor containing ytterbium (Yb), europium (Eu) as an activator (hereinafter referred to as GOS: Tb phosphor, GOS: Pr phosphor, GOS: Ce phosphor, GOS: Yb phosphor, GOS: Eu phosphor). Among these, a GOS: Tb phosphor having a high luminance and a GOS: Pr phosphor having a short afterglow are preferable, and a GOS: Tb phosphor is particularly preferable. Moreover, you may contain two or more of these activation substances.

The volume average particle diameter (also referred to as average particle diameter) of the phosphor contained in the phosphor layer is usually 30 μm or less, preferably 15 μm or less, and usually 0.01 μm or more, preferably 0.5 μm or more, more preferably 2 μm or more. More preferably, it is 5 μm or more.
Within the above range, it is preferable in that the effect of scattering light is large and the sensitivity of the obtained X-ray image conversion screen is good.
Further, the phosphor particles may be a mixture of two or more phosphor particles having different volume average particle diameters, or may be a mixture of three or more. By adjusting the particle size distribution of the phosphor particles, the sensitivity and DQE can be adjusted as appropriate.
As an example of a case where two or more kinds of phosphor particles having different volume average particle diameters are mixed, phosphor particles having a volume average particle diameter of usually less than 5 μm and 5 μm or more can be used.
The weight ratio of the phosphor particles having a volume average particle diameter of 5 μm or more to the total phosphor particles is usually 1 wt% or more, preferably 10 wt% or more, more preferably 30 wt% or more, still more preferably 50 wt% or more, particularly preferably It is 70 wt% or more, most preferably 80 wt% or more, and usually 100 wt% or less. The particle size difference between the phosphor particles having a volume average particle size of usually less than 5 μm and 5 μm or more is preferably 0.5 μm or more, more preferably 1 μm or more, further preferably 3 μm or more, and particularly preferably 5 μm or more. Usually, it is 100 μm or less, preferably 50 μm or less, more preferably 20 μm or less.

(Phosphor filling rate)
The filling rate of the phosphor in the phosphor layer is usually 40% or more, preferably 50% or more, more preferably 60% or more, and usually 100% or less.
Within the above range, it is preferable in that incident X-rays can be efficiently converted into visible light.
In addition, the measuring method of a filling rate is performed as follows.
The weight (W ₀ ) of the substrate before coating and the weight (W ₁ ) after forming the phosphor layer are measured. The weight per unit volume is calculated from the film thickness and area of the formed phosphor-containing layer and the measured film weight (W ₁ −W ₀ ).
On the other hand, the weight per unit volume of the phosphor contained in the layer is calculated from the ratio between the weight of the phosphor and the medium. From the calculated content per unit volume of the phosphor and the specific gravity of the phosphor, it is possible to calculate the filling rate (volume%) of the phosphor.

(Phosphor stacking amount of phosphor layer)
The amount of the phosphor layer of the phosphor layer (also referred to as phosphor amount) is not particularly limited, and can be appropriately set depending on the size of the X-ray image conversion screen and the required sensitivity. It is preferable to form a phosphor layer by laminating phosphor particles, usually 20 mg / cm ² or more, preferably 100 mg / cm ² or more, and usually 1000 mg / cm ² or less, preferably 500 mg / cm ^2. Hereinafter, it is more preferably 300 mg / cm ² or less. The X-ray image conversion screen according to the present invention is suitable for use with high-energy X-rays. In this case, the amount of phosphor layer is usually 110 mg / cm ² or more, preferably 120 mg / cm ² or more, more preferably 130 mg / cm ² or more, more preferably 140 mg / cm ² or more.
(Thickness of phosphor layer)
The thickness of the phosphor layer is not particularly limited, and is usually 10 μm or more, preferably 50 μm or more, more preferably 100 μm or more, still more preferably 150 μm or more, particularly preferably 200 μm or more, usually 1000 μm or less, preferably 600 μm or less, more preferably It is 500 μm or less, more preferably 450 μm or less.
Within the above range, the balance between sensitivity and sharpness tends to be improved.

[Method of forming phosphor layer]
The method for forming the phosphor layer is not particularly limited, and examples thereof include a method for forming a layer by a vacuum deposition method and a method for forming a layer by a wet film formation method.
Hereinafter, a method for forming a layer by a wet film forming method using the phosphor-containing composition will be described in detail.

  When forming a phosphor layer by a wet film-forming method, the process usually includes a phosphor-containing composition preparation step, a coating step, and a drying step. In addition, after a drying process, the post-processing process (process which performs washing | cleaning, drying, etc.) may be included arbitrarily.

(Phosphor-containing composition preparation process)
In the phosphor-containing composition preparation step, a phosphor-containing composition is prepared. The phosphor-containing composition may be a powder or a slurry.
The phosphor-containing composition contains the phosphor and may contain other medium, for example, a binder resin, a dispersant, a plasticizer, a photopolymerization initiator / thermal polymerization initiator, and the like as necessary. Moreover, in order to adjust the viscosity of a composition, the organic solvent etc. may be included.

The binder resin that may be contained in the phosphor-containing composition is not particularly limited as long as the effects of the present invention are not impaired. For example, nitrified cotton, cellulose acetate, ethyl cellulose, polyvinyl butyral, linear polyester, polyvinyl acetate , Vinylidene chloride / vinyl chloride copolymer, vinyl chloride / vinyl acetate copolymer, polyalkyl- (meth) acrylate, polycarbonate, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, gelatin, dextrin and other polysaccharides, gum arabic and the like.
When the phosphor-containing composition contains a binder resin, the binder resin is usually 0.1% by weight or more, preferably 2.0% by weight or more, and usually 20% by weight, based on the total amount of the phosphor-containing composition. Hereinafter, it is preferably 10% by weight or less.

Further, the dispersant is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include phthalic acid and stearic acid.
Furthermore, examples of the plasticizer include triphenyl phosphate and dibutyl phthalate.
One of these materials may be used alone, or two or more different materials may be used in combination.

  The organic solvent that may be contained in the phosphor-containing composition is not particularly limited as long as it can dissolve or disperse the phosphor and the medium. For example, ethanol, methyl ethyl ether, butyl acetate, ethyl acetate , Ethyl ether, xylene and the like. One organic solvent may be used alone, or two or more different organic solvents may be used in combination.

Note that the greater the difference in the refractive index between the phosphor and the medium, the greater the light scattering effect in the phosphor particles, and the refractive index of the medium is 1.3 in that light emitted from the phosphor is more likely to be collected in the recesses. The following is preferable, and 1.1 or less is more preferable.
A phosphor-containing composition can be prepared by appropriately mixing and stirring the raw materials containing the phosphor.

(Coating process, drying process)
In the coating step, the prepared phosphor-containing composition is coated on a support to form a coating film.
The method for forming a coating film by coating is not particularly limited as long as the effects of the present invention are not impaired, and known techniques can be applied. Examples thereof include a die coating method, a screen printing method, an ink jet method, and a spin coater method. It is done.
The formed coating film is dried or thermally cured using a hot plate, a hot air dryer or the like, or photocured using an ultraviolet irradiation device or the like to obtain a phosphor layer.
The phosphor layer may form a multilayer by laminating two or more different types of films. In the case of a multilayer structure, for example, layers having different average particle diameters and particle size distributions of phosphors or contained media can be appropriately laminated.

[Photodetector]
The photodetector includes a photoelectric conversion unit facing the phosphor layer, and has a function of converting fluorescence emitted from the phosphor layer into an electric signal or the like. The photodetector is not particularly limited as long as it has such a function, and a known photodetector can be used as appropriate.

[Adhesive layer]
The X-ray image conversion screen according to the present embodiment may have an adhesive layer between the dielectric multilayer reflective film and the phosphor layer. By including the adhesive layer, the trade-off curve of sensitivity and sharpness in the X-ray image conversion screen can be shifted in a desired direction.
The material of the adhesive layer is not particularly limited as long as the organic multilayer reflective film and the phosphor layer are bonded without impairing the effects of the present invention. For example, polyurethane resin, styrene-butadiene copolymer, acrylonitrile・ Butadiene copolymer, polyester resin, chloroprene resin, silicone resin, epoxy resin, etc., among others, from the viewpoint of adhesiveness and optical properties (sensitivity and sharpness), polyurethane resin, styrene / butadiene copolymer, acrylonitrile, A butadiene copolymer and a polyester resin are preferable, and a polyurethane resin is particularly preferable. The resin used as the adhesive may be the resin alone or may be a copolymer with other components as long as the present invention is not impaired. Further, within the range not impairing the present invention, for example, it may be modified with a carboxyl group, a sulfonic acid group, an amino group or the like, and a part of hydrogen of these functional groups may be replaced with an alkali metal or an alkaline earth metal. .

The adhesive used in the present embodiment may be classified into water dispersion, solution system, reaction system, hot melt system and the like, but is preferably an aqueous dispersion system or a solution system, and particularly preferably an aqueous dispersion system.
The adhesive layer preferably contains one or more resins, and one or more selected from the group consisting of polyurethane resins, styrene / butadiene copolymers, acrylonitrile / butadiene copolymers, and polyester resins. It is more preferable to contain this resin. The layer structure of the adhesive layer is not particularly limited, and may be formed of only one layer or may be a multilayer of two or more layers. For example, the upper surface of the organic multilayer reflective film may be composed of a polyurethane resin layer and a resin layer different from the polyurethane resin, or a mixed adhesive layer after previously mixing the polyurethane resin and a resin different from the polyurethane resin. It may be configured.

The method for applying the adhesive layer is not particularly limited as long as the effects of the present invention are not impaired, and a known technique can be applied. Examples thereof include a die coating method, a screen printing method, an ink jet method, and a spin coater method. Moreover, after apply | coating an adhesive agent, a drying process, a post-processing process (washing | cleaning, drying), etc. may be included arbitrarily.
The thickness of the adhesive layer is not particularly limited as long as the effect of the present invention is not impaired, but is usually 0.1 to 100 μm, preferably 0.5 to 30 μm, and more preferably 1 to 20 μm.
Within the above range, the adhesion between the organic multilayer reflective film and the phosphor layer is good, and optical characteristics such as sensitivity and sharpness are good.

[Protective film]
After forming the phosphor layer, a protective layer may be further formed on the phosphor layer.
The material for forming the protective layer is not particularly limited as long as the effects of the present invention are not impaired. For example, a radiation curable composition containing urethane (meth) acrylate, monofunctional (meth) acrylate, or polyfunctional (meth) acrylate. There are films of cured products, etc., and films such as PET having an adhesive layer. In the above-mentioned radiation curable composition, materials other than the above may be appropriately contained as required.

[X-ray inspection equipment]
In the X-ray inspection apparatus shown in FIG. 2, reference numeral 18 denotes a subject such as a human body, an animal, and various articles. The subject 18 is irradiated with X-rays 13 from a radiation source such as the X-ray tube 12. X-rays 13 absorbed or scattered by the subject 18 are irradiated to an X-ray imaging apparatus 15 that includes a flat panel detector 14. X-rays transmitted through the subject 18 are detected as an image signal by the flat panel detector 14. The image signal output from the X-ray imaging apparatus 15 is digitally processed by the image processing unit 16 and then displayed as an X-ray image (inspection image) on the display unit 17 such as a CRT.

  Hereinafter, the present invention will be described more specifically with experimental examples. However, the present invention is not limited to the following experimental examples without departing from the gist thereof.

[Production of X-ray image conversion screen]
(Experimental example A)
A binder was prepared by dissolving 20 parts by weight of a mixture of polyvinyl butyral resin, urethane resin fat and plasticizer in 80 parts by weight of a mixed solvent of toluene, 2-butanol and xylene, and stirring sufficiently.
The binder and the Gd ₂ O ₂ S: Tb phosphor having an average particle size of 9 μm are mixed sufficiently so that the resin amount and the phosphor amount of the phosphor layer after drying are in the weight ratio shown in Table 1. The mixture was stirred and further dispersed with a bead mill to prepare “phosphor composition 1”.
Next, a polyethylene terephthalate film containing calcium carbonate (UXQ2-188 manufactured by Teijin Ltd.) was used as a support substrate, and the “phosphor composition 1” using a blade coater, the gap between the support substrate and the blade at the time of application was determined. As shown in Table 1, a plurality of conditions were set and applied to form a phosphor layer on the support, and the phosphor layer was formed by drying at a drying temperature of 70 ° C. to 80 ° C. The thickness of the phosphor layer after drying and the phosphor coating weight were formed as shown in Table 1. In this way, the X-ray image conversion screen of Experimental Example A was obtained.

(Experiment B)
“Phosphor composition 2” was prepared in the same manner as in Experimental Example A except that the phosphor was changed to a mixture having a particle size shown in Table 1.
Subsequently, a phosphor layer was formed on the “phosphor composition 2” in the same manner as in Experimental Example A according to a plurality of conditions shown in Table 1. The thickness of the phosphor layer after drying and the phosphor coating weight were formed as shown in Table 1. Thus, the X-ray image conversion screen of Experimental Example B was obtained.

(Experiment C)
“Phosphor composition 3” was prepared in the same manner as in Experimental Example A except that the phosphor was changed to a mixture having the particle size shown in Table 1, and the phosphor amount and the resin amount were as shown in Table 1.
Next, in the same manner as in Experimental Example A, the “phosphor composition 3” was prepared using a polyethylene terephthalate film containing calcium carbonate (E60 manufactured by Toray Industries, Inc.) as a supporting substrate in accordance with a plurality of conditions shown in Table 1. Formed. The thickness of the phosphor layer after drying and the phosphor coating weight were formed as shown in Table 1. Thus, the X-ray image conversion screen of Experimental Example C was obtained.

(Experimental example D)
In the same manner as in Experimental Example A, a phosphor layer was formed using a polyethylene terephthalate film containing calcium carbonate (E60 manufactured by Toray Industries, Inc.) as a supporting substrate for the “phosphor composition 2” according to a plurality of conditions shown in Table 1. . The thickness of the phosphor layer after drying and the phosphor coating weight were formed as shown in Table 1. Thus, the X-ray image conversion screen of Experimental Example D was obtained.

[Setting of sensitivity-q'DQE standard line]
(Reference example)
Next, for each X-ray image conversion screen (X-ray scintillator screen DRZ series manufactured by Mitsubishi Chemical Corporation) shown in Table 2, an X-ray camera (Rad-icon manufactured by Teledyne Rad-icon Imaging) and an X-ray generator (( Sensitivity and q′DQE value were evaluated using PORTA 100HF manufactured by Job Co., Ltd.
Sensitivity and q'DQE values were measured with X-rays having a tube voltage of 80 kV and a tube current of 12 mAs through a water phantom with a thickness of 78 cm and a distance from the X-ray generation position to the X-ray camera light receiving surface. The sensitivity, MTF, and NNPS were measured, and the absolute value of the q′DQE value was determined and evaluated.
In the evaluation of MTF, NNPS and q′DQE values, the spatial frequency is a value at 2 lps / mm, as is normally done in Japanese Patent No. 3292548, Japanese Patent No. 3337103, etc. evaluated.
As a result, each X-ray image conversion screen shown in Table 2 has a trade-off relationship between sensitivity and q′DQE value in a high sensitivity region as shown in FIG. It was revealed. Further, when the approximate expression “y = ax + b” (least square method) of this straight line is obtained, it is expressed by the approximate expression “y = −29.222x + 174950” (a = −29.222, b = 174950). Became clear.

Further, for each X-ray image conversion screen of Experimental Examples A to D, the sensitivity and q′DQE value were evaluated in the same manner as in the above Reference Example. In the evaluation of the MTF, NNPS, and q′DQE values, the spatial frequency was a value at 2 lps / mm as described above, and the characteristics were evaluated. Further, the value of “y− (ax + b)” was obtained, and the improvement of the q′DQE value from the trade off-line was evaluated. The results are shown in Table 3 and FIG.
From the evaluation results, the X-ray image conversion screens of Experimental Examples A to C are based on combinations of phosphor particle diameter, particle diameter distribution, phosphor coating amount, reflectance of the support substrate on which the phosphor is coated, and the like. It has been clarified that an X-ray image conversion screen having higher sensitivity and q′DQE value than that of the X-ray image conversion screen can be obtained. A flat panel detector is created using an X-ray image conversion screen with high sensitivity and q′DQE value obtained from the X-ray image conversion screens of Experimental Examples A to C, and an image is acquired using an X-ray inspection apparatus. As a result, an X-ray image with high image quality was obtained according to the value of “y− (ax + b)”. In addition, it has been clarified that the effect is particularly remarkable in the high sensitivity region.
As is apparent from the figures and tables, the value of “y− (ax + b)” is preferably 5000 or more, and the value of “y− (ax + b)” is more preferably 7000 or more.
Since the X-ray image conversion screen of the present invention that satisfies the above requirements has high sensitivity, exposure by X-rays is small, and since the q′DQE value is high, a clear image can be obtained even at the same dose.

The definition of each item and the calculation method are shown below.
<Sensitivity>
Sensitivity was shown by the relative value of the average light quantity of 50 * 50 mm area detected with the X-ray camera when X-rays were irradiated to the X-ray image conversion screen.
The sensitivity may vary depending on the X-ray camera, but can be confirmed by normalizing the value of the DRZ series PLUS in Reference Example 2.
<Q'DQE>
q′DQE was calculated from the MTF and NNPS (normalized noise power spectrum) obtained by the following method using the following equation.
q'DQE = MTF ² / NNPS

(MTF)
An X-ray image was taken through a W (tungsten) edge having a thickness of 1 mm arranged at an angle of about 3 ° with respect to the pixel row of the X-ray camera, and ESF (edge spread function) was measured from the spread of the edge portion. LSF (line spread function) was calculated by differentiating the ESF, and MTF for the spatial frequency was calculated by further Fourier transforming the LSF.

(NNPS)
A noise image was obtained by subtracting the average light amount value of all pixels from the light amount value of each pixel in the X-ray image captured in the same manner as the sensitivity measurement. NPS (noise power spectrum) was calculated by performing a two-dimensional Fourier transform on the noise image and calculating the square of the absolute value. Further, NNPS was calculated by dividing NPS by the square of the above average light quantity value.
In the evaluation of the MTF, NNPS, and q′DQE values, the spatial frequency was a value at 2 lps / mm as described above, and the characteristics were evaluated.

<Reflectance>
The reflectance value measured by the following method is shown as a relative value, assuming that the reflectance of a standard white plate made of barium sulfate at a wavelength of 550 nm is 100%.
The reflectance was measured as follows. Using a spectrophotometer (Hitachi U-3310), the surface reflectance of the barium sulfate white plate was first measured under the conditions of a measurement wavelength of 200 to 800 nm, a scan speed of 300 nm / min, and a sampling interval of 1 nm. Calibrate the spectrophotometer. Next, the barium sulfate white plate is replaced with a sample and measured in the same manner. The reflectance at a wavelength of 550 nm of the measurement result was read.

1 Flat panel detector 2 X-ray image conversion screen 3 Photodetector
4 Support substrate 5 Phosphor layer 6 Protective film 11 X-ray inspection device 12 X-ray tube 13 X-ray 14 Flat panel detector 15 X-ray imaging device 16 Image processing unit 17 Display unit 18 Subject

Claims (9)

An X-ray image conversion screen including a support substrate and a phosphor layer laminated on the support substrate,
The phosphor layer contains a gadolinium oxysulfide phosphor having at least one selected from the group of Tb, Pr, Ce, Yb, and Eu as an activator,
An X-ray image conversion screen satisfying the following formula (1) in a sensitivity range of 1900 or more and 3600 or less when the sensitivity of the X-ray image conversion screen is x and q′DQE is y.
y − (− 29.222x + 174950) ≧ 5000 (1) The X-ray image conversion screen according to claim 1, wherein the amount of phosphor per unit area in the phosphor layer is 110 mg / cm ² or more. The X-ray image conversion screen according to claim 1 or 2, wherein the phosphor amount per unit area in the phosphor layer is 140 mg / cm ² or more.   The X-ray image conversion screen according to any one of claims 1 to 3, wherein a reflectance of a surface of the support substrate on which the phosphor layer is laminated is 90% or more.   The X-ray image conversion screen according to any one of claims 1 to 4, wherein a reflectance of a surface of the support substrate on which the phosphor layer is laminated is 95% or more.   6. The X-ray image conversion screen according to claim 1, wherein the support substrate contains at least one selected from the group consisting of titania, alumina, barium sulfate, calcium carbonate, zinc carbonate, and magnesium carbonate. N.   The X-ray image conversion screen according to any one of claims 1 to 6, wherein the phosphor contained in the phosphor layer has a volume average particle diameter of 5 µm or more and 15 µm or less.   A flat panel detector comprising the X-ray image conversion screen according to any one of claims 1 to 7 and a photodetector.   An X-ray inspection apparatus comprising the flat panel detector according to claim 8.

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